On Wednesday, "seanomccool" exposed the practice in a post on Reddit that has so far attracted more than 150 comments.

"I had an expensive, shitty pizza delivered to me last month that I ordered through Menulog," he wrote of his Melbourne experience. "I left a review saying the good and the bad. Good - size, speed. Bad - taste, very bad taste. It was an extremely fair review."

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He then received a call from Menulog on behalf of the restaurant owner asking him to pull down the review in exchange for a $15 voucher.

"I said, quite simply, no. I will not."

In an email to Fairfax Media, "seanomccool", who wanted to be known only as "Sean", said he had emailed Menulog on Wednesday morning complaining about the request, but was yet to receive a response.

"I used Menulog over services like Delivery Hero/Urbanspoon because I trust and believe the reviews left by customers," he wrote in the email to Menulog. "I didn't realise merchants could effectively buy an increased average rating."

Other Reddit users came forward with similar experiences.

"Shes_the_Cheese" said on Reddit that she had posted a "well constructed, polite but negative review" about a pizza eatery. Minutes later, the owner called her to offer free pizza if she removed the post.

"It's a gross breach of my privacy for her to call me about anything not pizza delivery related. ... She claimed the review was unfair and surprise! Menulog took it down! I contacted menulog and they said it was libellous for them to have a negative review on their website."

Freelance writer Julietta Jameson had to tell Menulog that her "opnion wasn't for sale" when the site forwarded a request from "Iwan", a venue manager at a restaurant in Melbourne, asking her to delete a post in exchange for a $10 voucher.

Averyll Enoka, Menulog's customer service manager, replied to her email, saying: "Yes it is true he does want it removed but our policy is to forward the request to the customer who left the review and if you agree we remove it if you do not we do not."

Consumer advocate Choice warned consumers not to rely solely on online user-generated reviews when choosing any product, including food.

"When it comes to user-generated reviews, it's a good idea to take them with a grain of salt because reviews are very easily manipulated," a Choice spokesman said.

The Australia Competition and Consumer Commission has taken legal action against a business that published fake reviews and testimonials online.

It says businesses that remove or edit for their benefit may risk misleading the public and breaching consumer law.